Liquid Crystal (LC) lenses, and a few other liquid crystal optical devices, are known in the art. One LC lens geometry has a planar construction in which liquid crystal material is held in a cell between glass or plastic plates. Usable optical lens power can be achieved within a relatively small LC cell thickness.
A variety of liquid crystal optical device designs have been proposed in which the orientation of the LC molecules changes in response to an applied electric field.
It has been shown that a circular hole patterned electrode placed at a distance above a LC layer, under which a uniform planar transparent electrode is located, can provide a LC lens by spatially modulating the electric field acting on such a LC layer. A GRIN lens can be created by controlling relative orientation of LC molecules creating a spatial variation of the index of refraction of the LC material within an aperture of the device.
In an article published by A. F. Naumov et al., entitled “Liquid-Crystal Adaptive Lenses with Modal Control”, OPTICS LETTERS/Vol. 23, No. 13/Jul. 1, 1998, a lens such as that shown in FIG. 1 uses an LC layer 10 positioned between a hole patterned electrode 14 located adjacent to a top glass support substrate 11, and a planar, optically transparent, electrode 12 of Indium Tin Oxide (ITO) located adjacent to a bottom glass support substrate 16. Liquid crystal alignment layers 18 are located to either side of the LC layer 10. The principle of operation of the LC lens of FIG. 1 relies on attenuating an electrical potential, and on a corresponding drop in electric field strength in the LC layer (across the hole patterned electrode aperture) between the periphery of the lens where the hole patterned electrode 14 is located, and the center of the lens. Since the typical thickness of LC layer 10 is about 0.05 mm, and typical optical apertures of interest are about 2 mm, i.e. forty times larger, left unaddressed, the radial drop in electric field strength across the LC layer 10 is drastic (rapid). A high resistivity layer 19 is deposited in the central part of the hole patterned electrode 14 to “soften” the drop in electric field strength by taking advantage of electrical signal attenuation provided by a distributed RC circuitry formed by the high resistivity layer 19 and the rest of the optical device layered structure. The resistance is provided mainly by the high resistivity layer 19 and the capacitance is provided mainly by the LC layer 10.
The GRIN lens of FIG. 1 is known to have some useful properties, but suffers from significant drawbacks. In particular, the operation of the lens is extremely sensitive to geometrical and material parameters of the layered structure. The most important of these is the sheet resistance Rs of the high resistivity layer 19, which is defined by Rs=(dσ)−1, where d is the thickness of the high resistivity layer 19 and a is its conductivity. This sensitivity complicates greatly the fabrication of a polarization independent Tunable Liquid Crystal Lens (TLCL) based on this technology:
The LC is a birefringent material. Incident light passing through the LC layer may be structured into two orthogonal light polarizations. The single LC lens layer 10 of FIG. 1 focuses a single polarization of light and leaving the other polarization essentially unaffected, therefore as the prior art lens uses a single LC layer 10, the overall LC lens optical device is polarization dependent. For this reason the general single LC layer geometry of FIG. 1 is generically referred to as a half LC lens.
Natural light (incoming from the sun or a lamp) contains a chaotic mixture of polarizations (which may be structured as a sum of two orthogonal polarizations). To provide a full polarization independent LC lens, one approach uses a combination of two half LC lenses, each having mutually orthogonal LC molecular orientation planes.
Two planar half LC lenses, each acting on a different light polarization, are arranged with the intent that each half lens focuses light of a corresponding polarity onto a common focal plane. In practice however, the ability to create different “polarization” LC lenses having identical optical properties is a great challenge. A full LC lens geometry that is too thick, with a large spacing between the two LC layers, results in a large spacing between focal planes of different polarization components and fails to create a clear natural light image due to each light polarization component being focused in different way with respect to a single plane optical sensor. In addition, when the lens shape and/or optical power of the two half LC lenses are not identical, the effect of each half LC lens is different even if the LC layers are positioned relatively close to each other. This difference may arise because of differences in LC layer thicknesses or differences in the sheet resistance values for the two high resistivity layers of the component half LC lenses which are combined to provide polarization independent operation.
The reduced camera sizes for mobile applications impose very difficult and strict requirements on camera design and lens performance. Accordingly, lens design must be carefully optimized taking into account both size and manufacturing cost considerations. In wafer-scale manufacturing, a wafer is produced containing a large number of half LC cells, and two such wafers are bonded together to make wafer level polarization independent full LC optical devices. However, for such separately wafer fabricated half LC lenses to have an identical optical power and lens shape when the two wafers are bonded to each other, the two wafers must have the same properties. While the thicknesses of the LC layers may be somewhat controlled by spacers, control of the sheet resistance of the high resistivity layer is a much more difficult task:
FIG. 2 illustrates one solution 200 proposed in PCT Patent Application WO/2009/153764, which is incorporated herein by reference, and which describes two orthogonally oriented LC layers 210a and 210b to focus orthogonal polarizations of light, arranged, respectively, above and below a common electric field control structure 326 having at least one middle ring hole patterned electrode 214(a, b) which is coated by a high resistivity material 219(a, b). Using a single (not shown) middle hole patterned electrode 214 provides a spatially modulated electric field for both the upper LC layer 210b and the lower LC layer 210a, with each of the two LC layers acting on a different polarization orientation of incident light. It was shown that the two LC lenses in such a geometry, image natural light in a substantially similar way onto the same imaging plane for example that of an image sensor. The spatial profile of the electric fields, and thus the optical power and aberrations, were shown to be essentially the same for both the upper 210b and lower 210a LC layers. In manufacturing, the bottom LC layer 210a has the hole patterned electrode 214a placed on top, and the top LC layer 210b is either fabricated on top (of the middle electrode structure 326) or separately fabricated (as illustrated in dashed line) and then bonded to the bottom LC layer 210a and hole patterned electrode 214a combination. Other 2xx series layers illustrated in FIG. 2 correspond to similar layers in FIG. 1 as described hereinabove, top half LC lens layers appearing in mirror fashion with respect to the bottom half LC lens. While not shown, each LC layer 210a and 210b is located between two liquid crystal alignment layers (see 18 in FIG. 1). Optically transparent conductive layers 212a and 212b are located between an alignment layer and a corresponding (support) substrate 216a and 216b. 
Using a hole patterned electrode 214, which employs a highly resistive layer (219a, 219b) of material, placed near the aperture, the electrical sheet resistance Rs of the material plays an important role in defining the shape of the electrical field and lensing properties, and such resistive properties are very important for precise control of the shape of the electric field inside LC layers (200). Controlling the resistance of a thin layer of a semiconductor material (219a, 219b) on a wafer within a required range for lensing operation using a 2 mm clear aperture is a challenge.
In addition, applications such as bar code reading, require auto-focus capability which means that an (electrically or otherwise) variable lens must be used to change the optical power of the overall camera. Such variable optical devices are referred to as Tunable LC Lenses (TLCL). This auto-focus capability destabilizes lens design optimizations introducing a degradation of a modulation transfer function (the transformation of an input optical image to an output optical image) which may be very severe (unacceptable).